The present invention relates generally to use of digital terrain elevation data (DTED), and particularly to a method of sampling digital terrain elevation data.
Digital terrain elevation data represents surface altitude at discrete "data posts." Each data post has a surface location or address, e.g., latitude and longitude, and an associated altitude, e.g., vertical offset relative to sea level. Thus, a simple form of a DTED database would deliver a scaler altitude in response to longitude and latitude input. More complicated DTED databases have been developed for certain applications. For example, U.S. Pat. No. 4,899,293 issued Feb. 6, 1990 to J. F. Dawson and E. W. Ronish shows a tessellation method for creating a spherical database by warping a digital map, including digital terrain elevation data, by longitude and latitude parameters.
DTED database systems are used in flight mission computer systems and flight planning strategy in military applications to aid in, for example, covert and evasive flight operations. As used in mission computer systems, a DTED database can aid a pilot in time-critical maneuvers or in selecting evasive routes with respect to a given threat position. Such threat positions may be known in advance and stored in the DTED database, or detected while in flight. The computation speed and accuracy required in accessing and calculating routes or alternatives based on DTED can be vitally critical, especially for computations executed repeatedly to keep a pilot fully appraised of current terrain conditions and route alternatives. Thus, improvements in methods of accessing DTED and computations based on extracted DTED are not simply improvements in computational elegance, but can be life-saving and critical to mission success.
There is often a need to perform sampling of a DTED database in a circular area surrounding a given point. Terrain masked threat intervisibility calculation is one example wherein a threat position is identified and a circular region surrounding the threat position is defined by the range capability of the threat. As may be appreciated, it is important to accurately extract a collection of DTED samples reflecting the actual range capability of the threat, i.e., extract a true circular data post pattern. Complete data post visitation or sampling of all data posts corresponding accurately to the circular threat range capability must be guaranteed.
As discussed more fully hereafter, the DTED database as applied to simple, dedicated hardware devices complicates the task of accurately extracting data posts within a circular region. More particularly, according to a prevailing data post model, i.e., that of U.S. Pat. No. 4,899,293 cited above, data post spacing is constant in the vertical direction along meridians or lines of longitude, but varies as a function of latitude, in the horizontal direction along parallels or lines of latitude. In other words, at latitudes near the poles data post spacing along lines of latitude is substantially less than post spacing along lines of latitude near the equator. Thus, data post spacing inconsistency or convergence complicates the task of extracting a circular pattern from a DTED database. Hardware devices proposed for use in executing such circular scan patterns relative to a DTED data cache, are simple, dedicated calculating engines imposing certain limitations on methods of executing the scan. For example, a constant radius and number of samples taken along each radial is typically required throughout the circular scan. This imposes a particularly troublesome requirement when attempting to scan a circular region from a database having variable data post spacing with such simple, dedicated scanning engine.